Scanning electron microscope

ABSTRACT

A scanning electron microscope includes an irradiation optical system for irradiating an electron beam to a sample; a sample holder for supporting the sample, arranged inside a sample chamber; at least one electric field supply electrode arranged around the sample holder; and an ion current detection electrode.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.11/230,642, filed Sep. 21, 2005, which claims priority from JapanesePatent Application No. 2005-041534, filed on Feb. 18, 2005, the contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Among means for obtaining secondary electron images by a scanningelectron microscope (hereinafter called “SEM”), this invention relatesto a method for forming images by utilizing gas ionization andmultiplication. Secondary electrons are multiplied in residual gasmolecules around, then ionized gas ions are detected. The inventionrelates to an electrode which provide electrostatic field and anelectrode which is utilized for ion current detection for improvingquality of the secondary electron images during high speed scanningacquired by SEM.

Various methods for multiplying the secondary electrons by utilizing gasionization and multiplication and then detecting the ion current havebeen attempted as image formation methods in SEM. The content of theprinciple of this kind is described in a paper “Secondary ElectronImaging in the Variable Pressure Scanning Electron Microscope”, Scanning20, 436-441(1998).

As a concrete application system of this means, JPA-2001-126655discloses a detection system that uses a sheet-like electrode as asecondary electron collector electrode during high vacuum secondaryimage observation and also as an electrode which provides electrostaticfield (referred as “an electric field supply electrode” hereinafter) anduses a sample holder as an electrode which is used for ion currentdetection (referred as “an ion current detection electrode”hereinafter). JP-A-2003-132830, on the other hand, discloses a detectionsystem that uses a secondary electron collector electrode as an electricfield supply electrode and a dedicated ion current detection electrodeseparate from a sample holder as an ion current detection electrode.JP-A-2002-289129 discloses a detection system that uses a sample holderas both of the electric field supply electrode and the ion currentdetection electrode.

FIG. 8 shows a basic construction of a detection system of SEM inJP-A-2003-132830. A primary electron beam 2 is converged by an objectivelens 6 onto a sample 14 put inside a gas atmosphere and a deflector 4two-dimensionally scans the sample 14. Secondary electrons 18 aregenerated from the sample 14 with the progress of irradiation of theprimary electron beam 2. The secondary electrons 18 are accelerated in adirection toward an electric field supply electrode 23 by the electricfield generated by this sheet-like electric field supply electrode 23 towhich a positive voltage is applied. The secondary electrons 18 therebyaccelerated collide with gas molecules around the sample and formelectron-ion pairs (gas ionization). The secondary electrons 18 and theelectrons generated by ionization are further accelerated by theelectric field generated by the electric field supply electrode 23,again collide with the gas molecules and form the electron-ion pairs. Asthis process is repeated, the number of electrons and the number of ionsincrease exponentially as they come close to the electric field supplyelectrode 23 (gas multiplication). The scale of this gas multiplicationis generally great when the drift distance of the secondary electrons(distance between sample and electric field supply electrode) is great.The ions drift towards the ion current detection electrode 22 that iselectrically connected to the sample holder 16 or is electricallyinsulated from the sample holder 16. The drifting ions are detected asan ion current. The resulting ion current signal is passed through anamplifier 19 and an A/D converter 31 and is used for image formation.The electric field supply electrode 23 shown hereby is used as asecondary electron collector electrode at the time of observation ofhigh vacuum secondary electron images.

FIG. 9 is an enlarged view of an electric field supply electrode 23 anda sample holder 16 of an electron microscope in JP-A-2001-12655. Thesecondary electrons 18 emitted from the sample surface create a largenumber of ions due to gas multiplication in the proximity of theelectric field supply electrode 23. The resulting ions 13 drift towardsthe sample holder 16 (ion current detection electrode) to which a groundpotential or a negative voltage is applied, and are detected as an ioncurrent from the sample holder 16. The current signal so obtained ispassed through an amplifier 19 and an AID converter 31 and is used forimage formation. The electric field supply electrode 23 hereby shown isused also as a secondary electron collector electrode at the time ofobservation of vacuum secondary images.

FIG. 10 is an enlarged view of portions in the proximity of a sampleholder 16 of an electron microscope in JP-A-2002-289129. The secondaryelectrons 18 emitted from the sample surface drift towards an objectivelens 6 kept at a ground potential by the electric field created by thesample holder 16 to which a negative voltage is applied, and generate alarge number of ions in the proximity of the objective lens 6 by gasmultiplication. The resulting ions are detected as an ion current fromthe sample holder 16. The resulting current signal is passed through anamplifier 19 and an A/D converter 31 and is used for image formation.

SUMMARY OF THE INVENTION

To improve the image forming speed and image quality in the gasmultiplication system ion current detection type SEM, it is necessary tosimultaneously accomplish improvement of the response speed of thecurrent signal by reducing the drift time, improvement of the ionmultiplication time by the electron avalanche and increase of the ioncurrent yield by improving detection efficiency of ions.

In JP-A-2001-126655 (FIG. 9), the space defined by the sample holder asthe ground potential and the electric field supply electrode to whichthe positive potential is applied is the drift space of the electronsand the ions. Since the electron avalanche exponentially increases inthe process in which the electrons drift inside the gas atmosphere, alarge number of ions occur at positions far from the sample holder (ioncurrent detection electrode). Therefore, the drift time in which theions occurring in the gas atmosphere reach the ion current detectionelectrode gets elongated and yet sufficient examinations have notnecessarily been made as to speed-up of the response speed of thecurrent signal used for the formation of the SEM image.

On the other hand, the greater the yield of the ion current, the greaterbecomes the improvement of image quality of the SEM image. The yield ofthe ion current is determined by the multiplication ratio of the ions bythe electron avalanche and by detection efficiency of the ion currentdetection electrode. It is known that the multiplication ratio of theions becomes generally greater when the drift distance of the secondaryelectrons becomes greater. For example, the means described inJP-A-2001-126655 (FIG. 9) and the means described in JP-A-2003-132830(FIG. 8) set the distance (drift distance) between the sample surface asthe secondary electron generation surface and the electric field supplyelectrode to a suitable distance and can optimize the ion multiplicationratio to a certain extent. According to the means of JP-A-2002-289129,however, the working distance (hereinafter called “W.D.”) results assuch in the drift distance of the secondary electrons and when W.D. issmall, a sufficient ion multiplication ratio cannot be obtained.

The ion multiplication ratio becomes greater when the potential gradientfor accelerating the secondary electrons becomes greater. Therefore, itcan be said that the ion multiplication ratio is dependent on thepotential gradient created by the electric field supply electrode, too.However, the means of the prior art described above do not sufficientlytake optimization of the shape of the electric field supply electrodeinto consideration for improving the ion multiplication ratio.

In a gas multiplication system ion current detection type SEM, SEMaccording to the invention includes at least one electric field supplyelectrode arranged around a sample holder and an ion current detectionelectrode so arranged as to cover the former.

According to such a construction, an SEM image can be acquired in thefollowing way, for example. The primary electron beam emitted andaccelerated from an electron gun is converged onto a sample by acondenser lens and an objective lens. An irradiation point of theprimary electron beam on the sample is scanned by a deflector with theconverging operation. The secondary electrons emitted from the samplewith the irradiation of the primary electron beam are multiplied byapplying a suitable voltage to the electric field supply electrode. Theions multiplied in this process are detected as an ion current from anion current detection electrode. Because the ion current detectionelectrode has the shape that covers the electric field supply electrode,the ions generated in the proximity of the electric field supplyelectrode can be efficiently detected. Because the distance between theelectric field supply electrode and the ion detection electrode issmaller than that of the electrode arrangement in the prior arttechnologies, the response speed owing to the drift time can beshortened.

In those SEM which form images from an ion current containing secondaryelectron information by utilizing secondary electron multiplication byresidual gas molecules around a sample, the invention can improve aresponse speed of a detection system and an ion yield in comparison withthe prior art technologies.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an SEM according to a first embodiment of the invention;

FIG. 2 shows a first example of an ion detection electrode of SEM and anelectric field supply electrode according to the invention;

FIG. 3 shows a second example of the ion detection electrode of SEM andan electric field supply electrode according to the invention;

FIG. 4 shows a third example of the ion detection electrode of SEM andan electric field supply electrode according to the invention;

FIG. 5 shows an SEM according to a second embodiment of the invention;

FIG. 6 shows an SEM according to a third embodiment of the invention;

FIG. 7 shows an SEM according to a fourth embodiment of the invention;

FIG. 8 shows a construction of SEM according to the prior art;

FIG. 9 in an enlarged view of an electric field supply electrode in theproximity of a sample holder;

FIG. 10 in an enlarged view of an electric field supply electrode in theproximity of a sample holder of SEM according to the prior art; and

FIG. 11 shows a typical measurement example of an ion current of SEMaccording to one embodiment of the invention.

FIG. 12 shows ion current characteristics in different kinds of gasaccording to one embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS Embodiment 1

FIG. 1 shows an SEM according to an embodiment of the invention. As anoperation of an irradiation optical system, a primary electron beam 2emitted from an electron gun 1 is converged by an objective lens 6 ontoa sample 14 positioned inside a gas atmosphere (typically, a gaspressure of 1 to thousands of Pa) inside a vacuum chamber 36 and thesample 14 is scanned by a deflector 4. Secondary electrons 18 aregenerated from the sample 14 with the irradiation of the primaryelectron beam 2. The secondary electrons 18 are accelerated by anelectric field created by a mesh-like electric field supply electrode 11(with typical mesh parameters of a mesh pitch of 1,250 μm and a meshwire diameter of 200 μm) to which a positive voltage (typically, 1 toabout 500 V) is applied and by a sample holder 14 to which a negativevoltage (typically, 0 to about −50 V) is applied. The secondaryelectrons 18 so accelerated impinge against gas molecules around thesample and form electron-ion pairs. The secondary electrons 18 and theelectrons generated by ionization are accelerated by the electric fieldcreated by the electric field supply electrode 11 and the sample holder16 and again ionize the gas molecules. As this process is repeated, theelectron avalanche occurs and the number of electrons and the number ofions increase exponentially as they come close to the electric fieldsupply electrode 11.

The ions drift towards an ion current detection electrode 7 that isgrounded, and are detected as an ion current. The resulting currentsignal is amplified by an amplifier 19 and is used for image formationthrough an A/D converter 31.

The ion current detection electrode 7 has an upper electrode and a lowerelectrode and these electrodes are kept at the same potential. Toimprove ion detection efficiency, a negative voltage (typically, −10 to0 V) may be applied to the ion current detection electrode 7.

An adjustment mechanism may be arranged so that the ion currentdetection electrode 7 and the electric field supply electrode 11interposed between the upper and lower electrodes while being coveredcan slide independently, and either manually or automatically, close toand away from the sample holder. These electrodes are moved eithermanually or automatically by a controller. When charging of the sampleaffects the image, the charge of the sample can be offset by controllingthe distance between the ion current detection electrode 7 or theelectric field supply electrode 1 and the sample 11 to thereby controlthe quantity of the ions or electrons approaching the sample holder.When the sample is charged to the negative by the irradiation of theprimary electron beam, for example, the quantity of the ions approachingthe sample surface is increased by moving the ion current detectionelectrode 7 away from the sample 14. The ions offset the negative chargeof the sample surface and reduce the charge of the sample surface. Inthis case, the yield of the ion current is controlled by adjusting thevoltage applied to the electric field supply electrode 11 and thedistance between the electric field supply electrode 11 and the sample14. When the sample 14 is charged to the positive by emission of thesecondary electrons, the electric field supply electrode 11 is broughtclose to the sample so that the major proportion of the electronsgas-multiplied approaches the sample surface. The electrons offset thepositive charge of the sample surface and reduce its charge. In thiscase, the yield of the ion current is controlled by adjusting thevoltage applied to the electric field supply electrode 11 and therelative distance between the ion current detection electrode 7 and thesample 14. When W.D. is changed, too, the distance between the samplesurface and each electrode can be kept constant by adjusting thepositional relation of each electrode, and the degree of the charge ofthe sample surface, the yield of the ion current and the drift distancecan be kept constant.

A voltage control mechanism 30 controls the voltages applied to thesample holder 16, the electric field supply electrode 11 and the iondetection electrode 7, respectively.

Optimization of the parameters for image observation such as a gaspressure, W.D., etc, is made by adjusting either manually orautomatically so that the ion current attains maximum.

The gas pressure is controlled by inputting a desired value to thecontroller (not shown). Control is made by opening and closing valveswhile the vacuum pump is operated. Optimization is conducted by applyinga high voltage to the electric field supply electrode 11 within therange in which discharge does not take place, and regulating the gaspressure. When it is desired to conduct measurement with higherresolution, it is necessary to shorten W.D. and to lower the gaspressure.

A positive voltage (typically, 1 to 500 V) is applied to the secondaryelectron collector electrode 23 when observation is made by setting thesample chamber 36 to a high vacuum state (typically, about 10⁻² to about10⁻⁵ Pa). In this case, a high vacuum secondary electron detector 8including a scintillator, an optical guide and a photomultiplier is usedfor the measurement of the secondary electrons.

FIG. 2 shows by an enlarged view an example of the ion current detectionelectrode 7 and the electric field supply electrode 11. In this example,the ion current absorption electrode 7 having a cylindrical shape coversthe periphery of a mesh type electric field supply electrode 11. In thisinstance, both ion current detection electrode 7 and electric fieldsupply electrode 11 may be allowed to independently slide towards andaway from the sample holder.

FIG. 3 shows another example of the ion current detection electrode andthe electric field supply electrode. In this example, a sheet-likedoughnut-shaped ion current detection electrode 25 (typically, having abracket shape) covers a mesh type doughnut-shaped electric field supplyelectrode 24 encompassing the sample 14.

FIG. 4 shows still another example of the ion current detectionelectrode and the electric field supply electrode. In this example, asheet-like ion current detection electrode 34 covers the periphery of amesh type electric field supply electrode 35. The sheet shape of the ioncurrent detection electrode 34 is such that it does not interfere withinsertion of other detector to be inserted around the objective lens 6.The ion current detection electrode 34 and the electric field supplyelectrode 35 are used as the secondary electron collector electrode forthe observation of the secondary electron image in high vacuum.

The shape of either one, or both, of the electric field supply electrodeand the ion current detection electrode is a single wire, a mesh, aporous plate or a sheet-like conductor.

According to such a construction, when the electric field supplyelectrode has the mesh shape having thin wires of a diameter of about200 μm, for example, electron avalanche is allowed to occur locally andeffectively by the electric field having a large potential gradient thatis generated in the proximity of this electrode. However, the shape ofthe electric field supply electrode is not limited to the thin wireshape hereby described.

Embodiment 2

FIG. 5 shows an SEM according to the second embodiment of the invention.In this detection system, a mesh type doughnut-shaped electric fieldsupply electrode 24 encompassing the sample 14 and a sheet-likedoughnut-shaped ion current detection electrode 25 (typically, having abracket shape) covering the doughnut-shaped electric field supplyelectrode 24 are used as the electric field supply electrode and the ioncurrent detection electrode in the same way as in FIG. 3. Here, the meshpitch is 1,250 μm and the mesh wire diameter is 200 μm as the meshparameters, for example. The doughnut-shaped electric field supplyelectrode 24 and the doughnut-shaped ion current detection electrode 25are fixed to the sample holder 16. The doughnut-shaped electric fieldsupply electrode 24 and the doughnut-shaped ion current detectionelectrode 25 may be fixed to a barrel. A positive voltage (typically, 1to about 500 V) and a negative voltage (typically, −10 to about 0 V) areapplied to the doughnut-shaped electric field supply electrode 24 andthe doughnut-shaped ion current detection electrode 25, respectively, tolet the secondary electrons drift towards the doughnut-shaped electricfield supply electrode 24 and to generate the electron avalanche in theproximity of the mesh wires of the doughnut-shaped electric field supplyelectrode 24. To impart the initial velocity to the secondary electrons,a negative voltage (typically, −50 to 0 V) is applied to the sampleholder 16.

An auxiliary electrode A10 interposed between the doughnut-like electricfield supply electrode 24 and the sample holder 16 is mesh-likeencompassing the sample 14 (typically, mesh pitch of 1,250 μm and meshwire diameter of 200 μm as mesh parameters). A positive voltage(typically, 1 to 200 V) lower than the voltage of the voltage supplyelectrode 24 is applied to the auxiliary electrode A10 to accelerate thesecondary electrons 18 towards the doughnut-shaped electric field supplyelectrode 24. The auxiliary electrode A10 is not limited to the meshshape as in the case of the electric field supply electrode.

The ions amplified by the electron avalanche are attracted towards thecurrent detection electrode 25 by the electric field and are detected asthe ion current from this electrode 25. The ion current so detected isamplified by the amplifier 19, is passed through the AID converter andis used for image formation.

The voltage control mechanism 30 controls the voltages to be applied tothe sample holder 16, the doughnut-shaped ion current detectionelectrode 25, the doughnut-shaped electric field supply electrode 24 andthe auxiliary electrode A10.

In the embodiment shown in FIG. 1, the distance between the sample 14and the ion current detection electrode depends on W.D. Therefore, theyield of the ion current depends on W.D., too, and each voltage and thegas pressure must be optimized whenever W.D. is changed. In thisembodiment, however, when the doughnut-shaped electric field supplyelectrode 24, the doughnut-shaped ion current detection electrode 25 andthe auxiliary electrode A10 are fixed to the sample holder at the timeof the change of W.D., they move simultaneously with the sample holder.Therefore, the distances between the sample 14 and the ion currentdetection electrode 24 and between the sample 14 and the doughnut-shapedion current detection electrode 25 do not depend on W.D. andre-optimization need not be carried out once each voltage and the gaspressure are optimized.

Embodiment 3

FIG. 6 shows an SEM according to the third embodiment of the invention.This embodiment uses electrodes having the shapes shown in FIG. 4 forthe ion detection electrode 34 and the electric field supply electrode35 and introduces an auxiliary electrode B26 (typically, wire shape ofdozens of μm). The major proportion of the secondary electrons generatedfrom the sample 14 drifts towards the electric field supply electrode 35but when a positive voltage (typically, 1 to 100 V) is supplied to theauxiliary electrode B26, the secondary electrons 27 that do not drifttowards the electric field supply electrode 35 drift towards theauxiliary electrode B26 and ionize the molecules in the residual gasduring their drift. The ions or electrons 29 generated by gasmultiplication drift towards the sample 14, offset the charge in thesample 14 and reduce the charge of the sample.

The formation method of the secondary electron image is the same asthose of SEM of the embodiments shown in FIGS. 1 and 5.

The voltage control mechanism 30 controls the voltage to be applied toeach of the sample holder 16, the ion current detection electrode 34,the electric field supply electrode 35 and the auxiliary electrode B26.

To observe the secondary electron image inside the sample chamber at ahigh vacuum, the ion current detection electrode 34 or the electricfield supply electrode 35 is used as the secondary electron collectorelectrode. The high vacuum secondary electron detector 8 detects thesecondary electrons collected by the secondary electron collectorelectrode. Because the ion current detection electrode 34 or theelectric field supply electrode 35 is used also as the secondaryelectron collector, each component can be mounted in a space savingarrangement.

Embodiment 4

FIG. 7 shows SEM according to the fourth embodiment of the invention.This embodiment uses the mesh-like electric field supply electrode 11similar to the electric field supply electrode 11 shown in FIG. 2 as theelectric field supply electrode and uses the sample holder 16 as the iondetection electrode. Here, the mesh pitch is 1,250 μm and the mesh wirediameter is 200 μm, for example, as the mesh parameters. Because thesample holder 16 is used also as the ion detection electrode, the ionsdrifting towards the sample holder 16 can be detected, too, anddetection efficiency can be improved. The secondary electrons 18generate the electron avalanche in the proximity of the electric fieldsupply electrode 11 and the multiplied ions drift towards the sampleholder 16 to which a negative potential (typically, −10 to 0 V) isapplied. The ion current detected from the sample holder 16 is amplifiedby the amplifier 33 inside the sample chamber, passes through the NDconverter 31 and is used for image formation. Because the amplifier isarranged inside the sample chamber, the distance between the iondetection electrode and the amplifier can be shortened and backgroundnoise can be reduced. The voltage control mechanism 30 controls thevoltages applied to the sample holder 16 and the electric field supplyelectrode 11.

To observe the secondary electron image at a high vacuum, the electricfield supply electrode 11, too, is used as the secondary electroncollector electrode in addition to the secondary electron collectorelectrode 23.

The modes of the detection system according to the invention are notlimited to only the four embodiments described above. For example, whenW.D. is great (15 mm or more, for example), the electric field supplyelectrode 11 and the ion current detection electrode 7 similar to thoseused in the first embodiment shown in FIG. 1 are used and when W.D. issmaller (10 mm, for example), the doughnut-shaped electric fieldelectrode 24 and the doughnut-shaped ion current detection electrode 25similar to those used in the embodiment shown in FIG. 5 are used. WhenW.D. is further smaller (5 mm or below, for example), the sample holder16 is used conjointly as the ion current detection electrode in the sameway as in the embodiment shown in FIG. 7, and all these modes areembraced in the scope of the invention.

Next, the results of a series of experiments conducted to compare thedetection system of the invention with that of the prior art will beexplained. FIG. 11 shows a typical measurement example of the ioncurrent by SEM of the embodiment of the invention shown in FIG. 1 and atypical measurement example of the ion current by SEM of the prior artshown in FIG. 9. The distance between the sample and the electric fieldsupply electrode 11 and the distance between the sample and the ioncurrent detection electrode are optimized to acquire the maximum yieldof the ion current. The ordinate represents the relative value of thecurrent measured by the ion current detection electrode 7 of theinvention (relative value of ion current detected at sample holder inaccordance with method disclosed in JP-2001-126655) and the abscissarepresents the voltage applied to the electric field supply electrode.The measurement is carried out at a beam acceleration voltage: 25 kV, abeam quantity: 200 pA, W.D.: 25 mm, a gas pressure: 100 Pa and a sample:copper material. In the measurement, the ion current detection electrode7 and the sample holder are kept at the ground potential. As shown inthe graph, the ion current greater by dozens of times than the ioncurrent quantity of the conventional detection system can be obtained interms of the maximum value in the embodiment shown in FIG. 1. Theresponse time can be improved by dozens of micro-sec owing to thereduction of the drift time of the ions. The residual gas inside thesample chamber is not limited to the atmosphere and a rare gas such asAr, Xe, etc, or a nitrogen gas may be used by separately providing gasintroduction means.

FIG. 12 shows the measurement results of the ion currents measured byusing the atmosphere, nitrogen, Ar and Xe, respectively. The gases otherthan the atmosphere are used as the residual gas in the followingconditions, for example. In low vacuum SEM observation under a lowvacuum condition of 1 to thousands of Pa, the degree of vacuum cannot beeasily elevated if vapor enters the sample chamber when the samplechamber is set to the low vacuum condition. Therefore, invasion of vaporinto the sample chamber is not desirable. For example, such a casecorresponds to the case where the degree of vacuum inside the samplechamber is again elevated. In such a case, it is advisable to use thenitrogen gas having voltage-current characteristics extremely analogousto nitrogen and atmosphere. Because Ar and Xe have a greater ionizationsectional area than the atmosphere molecules, a large ion current can beacquired at a relatively low voltage.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A scanning electron microscope comprising: an irradiation opticalsystem for irradiating an electron beam to a sample; a sample holder forsupporting the sample, arranged inside a sample chamber; an ion currentdetection electrode having a first area and a second area being face toface with the first area; and electric field supply electrode arrangedbetween the first area and the second area.
 2. The scanning electronmicroscope according to claim 1, wherein the ion current detectionelectrode has a cylindrical shape, and the first area and the secondarea are included inside of the cylindrical shape.
 3. The scanningelectron microscope according to claim 1, wherein the ion currentdetection electrode has an upper electrode and a lower electrode, andwherein the upper electrode includes the first area and the lowerelectrode includes the second area.
 4. The scanning electron microscopeaccording to claim 1, wherein the electric field supply electrode isarranged from the sample holder in a first direction, and wherein thefirst area is arranged from the second area in a second direction whichcrosses the first direction.
 5. The scanning electron microscopeaccording to claim 2, wherein the electric field supply electrode isarranged from the sample holder in a first direction, and wherein thefirst area is arranged from the second area in a second direction whichcrosses the first direction.
 6. The scanning electron microscope isarranged from the sample holder in a first direction, and wherein thefirst area is arranged from the second area in a second direction whichcrosses the first direction.
 7. A scanning electron microscopecomprising: an irradiation optical system for irradiating an electronbeam to a sample; a sample holder for supporting the sample; a firstelectrode including a first area and a second area being face to facewith the first area; and a second electrode arranged between the firstarea and the second area, and by which secondary electrons generatedfrom the sample with the irradiation of the electron beam areaccelerated.
 8. The scanning electron microscope to claim 7, whereinvoltages are applied to the first electrode and the sample holder suchthat the secondary electrons are accelerated.
 9. The scanning electronmicroscope according to claim 7, wherein the first electrode is adetector for detecting ions generated by impingement of the secondaryelectrons accelerated against a residual gas inside a sample chamber.10. The scanning electron microscope according to claim 7, wherein thefirst area and the second area are arranged in a direction which anacceleration direction of the secondary electrons crosses.
 11. Thescanning electron microscope according to claim 7, wherein the secondelectrode has an upper electrode and a lower electrode, and wherein thesecond electrode has an upper electrode and a lower electrode, andwherein the upper electrode includes the first area and the lowerelectrode includes the second area.
 12. The scanning electron microscopeaccording to claim 1, wherein the first electrode has a cylindricalshape, and the first area and the second area are included inside of thecylindrical shape.
 13. A scanning electron microscope comprising: achamber; a sample holder to support a sample in the chamber; anirradiation optical system to irradiate an electron beam to the sample;an electrode to accelerate secondary electrons generated from the samplewith the irradiation of the electron beam; and a detector having a firstarea and a second area which face to each other in a direction which anacceleration direction of the secondary electrons crosses.
 14. Thescanning electron microscope according to claim 13, wherein voltages areapplied to the electrode and the sample holder such that the secondaryelectrons are accelerated.
 15. The scanning electron microscopeaccording to claim 13, wherein the detector detects ions generated byimpingement of the secondary electrons accelerated against a residualgas inside a sample chamber.
 16. The scanning electron microscopeaccording to claim 13, wherein the detector has an upper electrode and alower electrode, and wherein the upper electrode includes the first areaand the lower electrode includes the second area.
 17. The scanningelectron microscope according to claim 13, wherein the detector has acylindrical shape, and the first area and the second area are includedinside of the cylindrical shape.